Electrophotographic image forming apparatuses are widely used, in which an electrostatic latent image is formed on a photoconductor (photoconductive drum) and toner is applied onto the latent image to form a toner image and the toner image is transferred and fixed onto paper. Particularly, a wet development method using liquid developer, which has relatively small size toner particles and seldom causes irregularity in images, is beginning to be used in image forming apparatuses required to have higher image quality and image resolution such as office printers for printing a lot of documents and on-demand printing apparatuses.
Recently, wet development devices and image forming apparatuses using a high viscous and high concentration liquid developer, which is prepared by dispersing solid toner component composed of a resin and a pigment in high concentration in an insulating liquid (carrier liquid) such as silicone oil, have been proposed.
When the development is carried out by using such liquid developer, it is usual that a micron order thin layer of developer is formed on a developer carrying member such as a developing roller and the thin-filmed developer is brought in contact with the photoconductor to develop.
In a common wet development method, the thin layer of the liquid developer is formed on the developing roller by a thin layer forming method in which an amount of carried developer is regulated with a blade, and the toner particles in the thin developer layer are then charged by a charging section provided on the upstream side in the developer carrying direction, and the latent image formed on the photoconductor in a facing position is developed in the development area.
However, the liquid developer is usually left on the developing roller after the development of the latent images on the photoconductor. When such remaining developer is again arrived at the development area, it creates a bad influence on the next image such as memory phenomenon.
Therefore, a technique for cleaning the developer remaining on the developing roller has been developed.
To clean the developer remaining on the photoconductor, it is usual that the developer is scrubbed off with a blade touching the photoconductor. Therefore, a technique by touching a blade to the developing roller for scrubbing off the developer remaining on the developing roller has been applied.
In the case of the developing roller, however, it is difficult to use a material of high hardness for the surface of the developing roller in view of the parts for supplying the developer and the photoconductor in contact. Thus, if the touching pressure of the blade is increased, the durability of the developing roller is decreased, and if the touching pressure is too low, the remaining developer is not satisfactory cleaned with the developer slipping through the blade.
To address the above problems, Japanese Laid-Open Patent Application Publication No. 2005-345932 discloses a technique for easily scrubbing off the toner by the blade, in which a toner releasing member is provided facing the developing roller, after the development area and before the cleaning area, and voltage is applied between the toner releasing member and the developing roller for electrically releasing the toner in the developer from the developing roller surface, so that the toner is easily scrubbed off by the blade.
In addition, Japanese Laid-Open Patent Application Publication No. 2003-225893 discloses a technique in which the toner is absorbed and removed by a cleaning roller applied with a bias voltage, before cleaning by the blade.
The removal of the remaining developer after development on the developing roller is influenced by the charge of the toner in the developer. To put it into anther words, when the toner is highly charged, the adhering force to the developing roller is accordingly high and the cleaning is accordingly difficult by the mechanical scrubbing force of the blade.
On the other hand, the highly charged toner is advantageous for inhibiting fog and image noise created at the time of development. Furthermore, the amount of adhered toner (image density) and the gradation (dependence of the toner adhering amount on the potential difference) can be controlled by changing the charging amount of the toner.
In the techniques disclosed in Japanese Laid-Open Patent Application Publication Nos. 2005-345932 and 2007-225893, the toner is electrically removed, after development, by the toner releasing member before the cleaning by the blade, but they have a problem that the cleaning cannot satisfactorily carried out since any measure to deal with variation in the charge of the toner is not provided.